Technology for utilization of coal which is cheap and abundant is being widely developed due to the price rise of crude oil and natural gas. In particular, Synthetic Natural Gas (SNG) technology is methods for producing fuel gas which includes methane as a main component by using a solid fuel such as coal, etc., and is satisfactory from the aspects of energy security and environmental issue of use of energy. In addition, the SNG technology can use a natural gas infrastructure as it is and thus may be the most realistic clean coal technology along with technology of Integrated Gasification Combined Cycle (IGCC).
From among the methods for producing SNG, an indirect method for obtaining SNG from syngas obtained by gasifying coal through a methane synthesis reaction using a catalyst is widely used. This method goes through a production process shown in FIG. 1, for example.
Referring to FIG. 1, first, syngas including carbon monoxide (CO) and hydrogen (H2) as main components is obtained by gasifying coal in a coal gasifier 1. Since a H2/CO ratio of the syngas discharged from the coal gasifier is below 1.0, a water gas shift reaction is performed in a water gas shift reactor 2 to increase a H2 concentration to make the H2/CO ratio reach about 3.0. This reaction is an exothermic reaction and has difficulty in controlling heat of reaction. Therefore, when temperature of a catalyst excessively increases, original activation is lost and a water gas shift reaction may not occur. In this case, it is common that the H2/CO ratio is adjusted to be about 3.0 by passing only 30% of the syngas through the water gas shift reactor and bypassing the remaining syngas.
The syngas which has gone through the coal gasification and water gas shift reaction is cooled in a cooling device 3, sulfide (e.g., H2S, COS) and carbon dioxide (CO2) are removed in a desulfurization/CO2 separating device 4, and then the syngas is introduced to a methanation reactor 5.
In the methanation reactor 5, methane is synthesized by a reaction of CO and H2. The methane synthesis reaction is a strong exothermic reaction and high pressure condition is favorable thermodynamically. However, as reaction temperature increases, a methane yield is reduced. Therefore, extracting and controlling heat of reaction effectively to obtain a high methane yield are the most important factor in designing a methanation reactor.
For example, Korean Patent Publication No. 2013-0095647 discloses controlling such heat of reaction. This disclosure suggests a method which uses a plurality of reactors in a methane synthesis process, and controls heat of reaction by re-circulating products discharged from some reactors to the former reactor, mixing the products with newly introduced gas, and reducing concentrations of CO and H2 of supplied gases. In this method, however, a methane composition of 90% or more can be obtained only if at least 3-stage or 4-stage reactors are used, and there is a problem that many reactors, a heat exchanger, and a gas re-circulation system are required to control heat of methanation reaction.
In addition, Korean Patent Publication Nos. 2013-0075549 and 2014-0084486 propose a method using a reactor which has a heat exchanger installed inside or outside a catalyst layer. However, this method has disadvantages that a structure is complicated and it is difficult to replace a catalyst when activation of a catalyst is degraded. In addition, since most of the methanation catalysts is manufactured in the form of a tablet or pallet, the methanation catalyst is used in a fixed bed type reactor, and heat generated by the reaction is stored from the starting region when syngas is supplied to the fixed bed reactor. Therefore, there is a problem that temperature is not uniformly distributed over the reactor.